Adam Brown: belief

Transcript context

…Can you tell the story of how GR went from a theory that Einstein had to something that the world came to believe is true? That would be the bending of light. There were known anomalies with Newton’s physics beforehand, like we couldn’t get the orbit of Mercury exactly right. One of the very nice early tests of general relativity is that it did get the orbit of Mercury exactly right. So that was a pretty good confirmation. But at the same time, that’s not quite so satisfying because it was a number that’s already known, as opposed to one where you invent a theory and then it correctly predicts. It’s considered more impressive if you get the right answer without knowing what the right answer is in advance. So that would be the bending of light. Certainly, historically, that was the most influential. According to general relativity, all energy gravitates and all energy is affected by gravity. So light, as it’s passing a massive object like the Sun, will get bent in the direction of the Sun. Actually, the same will happen in Newtonian physics. Suppose you have a particle going along. You know how much it gets bent as it passes the Sun, depending on its impact parameter, but also its velocity. The faster it’s going, the less it gets bent. So you just take that Newtonian formula, plug in velocity equal to speed of light, and see what answer you get. You can get a certain amount of bending through Newtonian physics. In general relativity, you can do the same calculation, and you actually get double the Newtonian answer. The slightly strange history of it. Before he had finished writing down general relativity, Einstein had a prediction based on his understanding of the equivalence principle for what this answer should be. He wrote down the answer, and then in response to him and a number of other people being interested in this, people were sending out expeditions to go and try and measure it. I think the very first thing that he did was he phoned up the observatory and said, “Can you look at distant stars behind the Sun and measure how light bends as it passes the Sun?” This was the true theorist move, because I think the director of the Mount Wilson Observatory said, “Absolutely not. We cannot do that. If you point a telescope at the Sun, you’ll go blind. If you point it just next to the Sun, you’ll just get washed out by the corona of the Sun and you won’t see anything.” Except there’s one time when you won’t get washed out by the Sun, and that’s during a total solar eclipse, when the Moon blocks the Sun and you’re able to see stars very close to the Sun and measure the bending of the light behind them. So during the 1910s, there were a whole bunch of expeditions sent to measure the deflection of light. They’d park out in the path of totality and look through telescopes at the stars right next to the Sun and see if they moved in the sky, and if they moved, how much they moved. I think the first one was in 1911. They went to Argentina for an eclipse, and everything’s set up. This is the problem with this thing. f they moved in the sky, and if they moved, how much they moved. I think the first one was in 1911. They went to Argentina for an eclipse, and everything’s set up. This is the problem with this thing. You get there, all the way to Argentina, a very long way in those days, and then it’s just washed out by the clouds. You don’t see anything, and it’s very frustrating. The next one was a German expedition sponsored by the arms manufacturer Krupp, who went to the Crimea and tried to measure it there. Just before the solar eclipse happens, World War I breaks out, and now Germany and Russia are at war. They’re all arrested and turned for the rest of the war, and so that also fails. It actually turns out to be a good thing for Einstein that they all failed. It turned out that Einstein’s original equivalence principle argument—before he had full general relativity—was wrong and led him to predict that the bending of light in general relativity would be the same as it was in Newtonian physics. During the war, while everything is shut down and no one is thinking about eclipse expeditions, he corrects this mistake and comes up with a new prediction that actually it’ll be double the Newtonian prediction. And then in 1919, Sir Arthur Eddington launches a British expedition to go and observe the eclipses all over the world and successfully comes back and declares that indeed it was the Einstein prediction. It was double the Newtonian prediction. That’s really what launches Einstein as a global celebrity. This British experiment confirming a German-origin theory was part of the post-war reconciliation, and Einstein had figured out everything. That is, I’d say, the point at which general relativity became the consensus view, and people were super convinced by this very impressive test. Nowadays, we’ve done hugely more tests than that, very precise orbital dynamics. You can see it in the orbit of Mercury and indeed even in the other planets. You can just measure the redshifting of light as it goes, the gravitational effect on the propagation of light or the energy of light, all over the place. But historically, that was the most impressive confirmation of general relativity.…

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